1. Field of the Invention
This invention relates, in a first of its aspects, to a method of and an apparatus for applying a coating to articles, eg an edible coating to food articles. The invention is particularly, but not exclusively, concerned with the application of edible coating materials which exhibit non-Newtonian behaviour, for example chocolate, to articles of confectionery and the like. The present invention also relates, in a second of its aspects, to an improved method of forming a curtain of material eg edible material which can be used to coat articles eg articles of food or which can be used in other ways in the formation of articles, eg articles of food.
2. Description of Related Art
It is well known to enrobe articles of food such as chocolate assortmnents, confectionery bars, biscuits, cookies and cakes with a layer of chocolate. This coating process is known as chocolate enrobing and is traditionally effected by moving the articles on a mesh-type conveyor belt through a curtain of liquid chocolate whose consistency is carefully controlled. However, it is difficult to achieve the desired coating because of the high viscosity. It is therefore common practice to subject the articles to a greater than needed coating, then controlling the amount of chocolate remaining on the articles by blowing by air from fans and using vibration to remove the excess.
Also, the temperature of chocolate cannot be increased so as to reduce its viscosity as it will lose its temper, causing problems of incorrect fat crystallisation which can deleteriously affect the appearance and/or eating quality of the chocolate. Thus, it is common for enrobing chocolate to have a higher fat content than standard chocolate used for making chocolate bars in order to reduce its viscosity. This has adverse cost implications.
In order to establish the curtain of chocolate, it is known to allow chocolate to pass under the action of gravity through an outlet slot in the bottom of a trough having inclined side walls leading to the slot. This type of apparatus has a relatively low throughput because of the high viscosity of the chocolate and the relatively low rate at which it can flow through the outlet slot. The common solution to this problem is to use a wider slot, which results in a thicker curtain which then requires removal of more excess from the articles. Because of the physical characteristics of the chocolate which passes through the slot, the curtain can xe2x80x9cneckxe2x80x9d to a substantial extent. In other words, the width of the chocolate curtain becomes substantially less than the length of the slot from which it issues. This means that the effective cover of the curtain over the width of the conveyor belt used to carry the articles through the curtain is reduced.
Another known form of apparatus for producing a chocolate curtain utilises a roller along which liquid chocolate is distributed and carried to a blade which causes the layer of chocolate on the roller to become detached and thereby establish the curtain. This can provide higher coating rates with a better control of curtain thickness, but difficulties still arise in coating.
With both of the above types of known apparatus, the amount of chocolate in the descending curtain is greatly in excess of that required to coat the articles. The excess drains through the mesh-type conveyor belt and needs to be recycled and its condition carefully controlled.
It is an object of a first aspect of the present invention to obviate or mitigate at least some of the above disadvantages.
In accordance with said first aspect of the present invention, there is provided a method of applying a coating to articles, comprising the steps of:
(i) providing a curtain of solidifiable liquid coating material;
(ii) effecting relevant movement between articles to be coated and the curtain so as to coat the articles with the solidifiable liquid coating material; and
(iii) subjecting the solidifiable liquid coating material to the action of at least one stream of gas under pressure whereby to modify the flow characteristics of the curtain.
Also in accordance with said first aspect of the present invention, there is provided apparatus for applying a coating to articles, comprising:
(i) supply means arranged to provide a curtain of solidifiable liquid coating material;
(ii) means for effecting relative movement between articles to be coated and the supply means whereby in use the articles are coated with the solidifiable liquid coating material in the curtain; and
(iii) means arranged to subject the solidifiable liquid coating material to the action of at least one stream of gas under pressure whereby to modify the flow characteristics of the curtain.
Preferably, the articles are articles of food and the coating material is an edible coating material.
In the case where the present invention is used in a coating apparatus of the type in which the coating material is caused to flow along a surface of the supply means (eg a trough) towards an outlet slot through which the coating material flows under the action of gravity to form the curtain, it is preferred for said at least one stream of gas to be introduced between the coating material and the surface of the supply means. In this way, a layer of gas can be introduced between the surface and the coating material as it flows towards the outlet slot. Preferably, in the case of a trough where the coating material flows over opposed surfaces towards the outlet slot, a layer of gas is introduced between the coating material and each of the opposed surfaces. This can not only substantially reduce the resistance to flow of the coating material over the surfaces, but can also reduce the viscosity of the coating material if it is of a type whose viscosity is reduced when subjected to shear.
It is particularly preferred to cause said at least one stream of gas under pressure to become attached to the surface of the supply means so as to assist in establishing the layer of the gas between the surface and the coating material. This may be achieved by positioning one or more gas-admission slots in such a way as to direct the gas against the surface of the supply means. The spacing between the outlet slot and the or each gas-admission slot depends upon the nature of the coating material and the geometry of the supply means, which may comprise a trough having V-shaped walls defining opposed surfaces which converge towards the outlet slot. If the or each gas-admission slot is disposed too close to the outlet slot, then the flow of gas through the outlet slot may actually restrict the flow of coating material therethrough. On the other hand, if the or each gas-admission slot is disposed too far away from the outlet slot, the coating material flowing over the surface may become re-attached to the surface of the supply means before it reaches the outlet slot.
In the case where the above-mentioned trough is employed, it is within the scope of the present invention to provide said at least one stream of gas under pressure at one or both convergent opposed surfaces of the trough leading to the outlet slot.
Said at least one stream of gas under pressure may be applied to the coating material after the curtain has been established in order to change the direction of the curtain and/or a physical property of the coating material forming the curtain. It is also within the scope of the present invention to subject the coating material to the action of at least one stream of gas both before and after the curtain has been established.
A curved surface may be provided adjacent to part of the curtain, and means may be provided for causing a stream of gas to flow over the curved surface by virtue of the Coanda effect and to use this to induce a change in the direction of travel of the curtain. In this way, it is possible to control the direction of flow of the curtain from any angle from vertical to substantially horizontal. This effect can be used whether or not the coating material is subjected to the action of at least one stream of gas under pressure before the curtain is established. In this regard, the curtain may be established by flow of the coating material through an outlet slot or by distributing the coating material over the length of a roller and causing it to be transported to a blade which removes the coating material from the roller and thereby establishes the curtain.
It is within the scope of the present invention to use one or more curtains of coating material to coat the articles and to control the direction of movement of these curtains simultaneously or independently in such a way as to ensure maximum coverage. For example, one of the curtains may be controlled so that its direction of movement is an acute angle (e.g. 45xc2x0) relative to the direction of relative movement between the articles and the curtain, whilst the other curtain can be controlled so that its direction of movement is at an obtuse angle (e.g. 135xc2x0) with respect to said direction of relative movement. In this way, coating of upstream and downstream ends of the articles may be facilitated.
In certain embodiments, the control of the pressure of the stream of gas can be employed to control the speed of descent of the curtain. Thus, by controlling the rate of descent of the curtain and the rate of relative movement between the articles and the curtain, a variety of different effects can be achieved. For example, if the rate of descent of the curtain is matched with the rate of relative movement, then a smooth coating can be achieved. If the rate of curtain descent is greater than the rate of relative movement, then a surface patterning effect can be achieved by the resultant folding of the applied curtain onto the articles. On the other hand, if the rate of descent is less than the rate of relative movement, a degree of stretching of the coating material as it becomes attached to the articles may be achievable with resultant thinning of the layer of coating material, applied to the articles. The effects achieved will also depend upon the physical properties of the coating material.
The present invention is applicable to the use of non-Newtonian liquids such as chocolate or Newtonian liquids such as caramel as coating materials.
In the case of chocolate (or other non-Newtonian liquid, a liquid whose viscosity reduces when subjected to shear), a very high degree of control is achievable because the stream of gas under pressure can be caused to contact the surface of the chocolate in such a way as to reduce its viscosity by application of a shear force. This has the particular advantage that the chocolate flows much more easily but then rapidly thickens once the shear force has been removed. The application of the stream of gas under pressure to the chocolate before the curtain has been established can enable the previously mentioned necking problem to be mitigated and can also enable a much higher throughput to be achieved for a given size of coating apparatus. Because of the viscosity reduction achieved, it is possible to coat with a much higher viscosity chocolate than has heretofore been considered possible. For example, it is possible to coat with relatively viscous tempered chocolate, rather than having to coat with a chocolate composition having an increased fat content and subsequently lowered tempered viscosity in order to establish and maintain the desired liquid curtain. The need to effect air blowing and/or vibration on the coated articles may be obviated or mitigated. The present invention permits a curtain of even thickness to be achieved and may also enable a thinner curtain to be produced than has heretofore been possible.
Conveniently, the gas is air. The temperature of the gas may be substantially the same as that of the solidifiable liquid coating material. This is particularly advantageous in the case where the material is liquid chocolate.
It will be appreciated that the present invention in its first aspect involves the control of a curtain of solidifiable liquid coating material for the purpose of coating or enrobing articles. However, it will be appreciated that similar techniques can be employed for controlling a curtain of a solidifiable liquid material for use in the production of other articles. For instance, the curtain can be controlled for the purpose of enabling a layer of the material to be deposited into a mould or moulds (e.g. to produce shells of solidified material which can then be used to contain filling material), or onto a conveyor for solidification as a layer thereon which can be subsequently cut to size or otherwise shaped. The fact that the curtain can be very accurately controlled in terms of the thickness of the curtain and/or its angle/speed of descent means that a close control over the thickness and/or texture of the deposit can be obtained which can be difficult to achieve with standard confectionery shell technology.
Thus, in its second aspect, the present invention resides in a method of controlling a curtain of a solidifiable liquid composition comprising the steps of:
(i) providing a curtain of solidifiable liquid material; and
(ii) before, during and/or after step (i), subjecting the solidifiable liquid material to the action of at least one stream of gas under pressure whereby to modify the characteristics of the curtain.
Also in accordance with said second aspect of the present invention, there is provided apparatus for controlling curtain of a solidifiable liquid composition comprising:
(i) supply means arranged to provide a curtain of solidifiable liquid coating material; and
(ii) means arranged to subject the solidifiable liquid coating material to the action of at least one stream of gas under pressure whereby to modify the characteristics of the curtain.
The method and apparatus may further include provision for (a) depositing the modified curtain of solidifiable liquid composition to form a layer in a mould or on a surface, and (b) solidifying the deposited composition.
The solidifiable liquid composition is preferably an edible composition.